1. Field of the Invention
The present invention relates to a signal demodulating apparatus for use with video disc players that reproduce data from video discs having video signals recorded thereon in the so-called MUSE (multiple sub-nyquist sampling encoding) format for the Japanese high-definition television known as Hi-vision.
2. Description of the Prior Art
FIG. 1 is a block diagram of a recording apparatus for recording Hi-vision video signals in the MUSE format onto video discs. In FIG. 1, an FM modulator 51 frequency-modulates input video signals on a 12.5 MHz carrier by an input video signals and supplies an FM signal fc to an adder 52. The frequency deviation of the FM signal fc is .+-.1.9 MHz, and the black and white levels thereof are 10.6 (=12.5-1.9) MHz and 14.4 (=12.5+1.9) MHz, respectively. The adder 52 is also fed with a pilot signal fl at 2.3 MHz (=f.sub.H .times.135/2; where f.sub.H stands for the horizontal scanning frequency). The pilot signal is used in TBC (time base corrector) and spindle servo upon playback. The adder 52 adds the FM signal fc and pilot signal fl and supplies the result to a limiter 53. In turn, the limiter 53 limits the amplitude of the received signal to a predetermined level. The output signals from the limiter 53 are supplied to a laser disc cutting machine that records the signals onto video discs. FIG. 2 is a view of the frequency allocation of MUSE-format signals recorded on video discs.
When the limiter 53 limits the amplitude of the mixed signal composed of the FM signal (main carrier) fc and pilot signal fl, there are generated a lower sideband fp and an upper sideband fp+. That is, as shown in FIG. 3 (A), addition of the pilot signal fl to the FM signal fc causes a vector F1 related to the pilot signal fl to rotate around the tip of a vector Fc of the FM signal fc in accordance with the phase change in the pilot signal fl. Consequently, the mixed signal has not only the phase change (i.e., frequency-modulated component) but also the amplitude change (i.e., amplitude-modulated component).
When the amplitude of the mixed signal is limited to a predetermined level, as depicted in FIG. 3 (B), the vector F1 is formed from two vectors: vector Fp related to the lower sideband fp, and vector Fp+ related to the upper sideband fp+. Even if the vectors Fp and Fp+ rotate in accordance with the phase change, the vector F1 does not rotate; a length of the vector F1 varies in the direction perpendicular to the vector Fc. At this time, the amplitude of the vectors Fp and Fp+ becomes half that of the vector Fl. Both the lower sideband fp and the upper sideband fp+ are recorded on the video disc together with the FM signal fc.
When the frequency of the FM signal fc is 10.6 MHz (i.e., when the picture appears blackish), the frequency allocation of the mixed signal is as shown in FIG. 4. And when the mixed signal is supplied the limiter 53, the upper sideband fp+ at 18.9 (=10.6+8.3) MHz is generated, as illustrated in FIG. 5. That frequency is higher than the frequency fc (=10.6 MHz) of the main carrier by 8.3 (=10.6-2.3) MHz, i.e., by the difference between the frequency of the main carrier fc and that of the pilot signal fp. A signal phase of the upper sideband fp+ is opposite to a signal phase of the lower sideband fp.
FIG. 6 is a view showing the frequency band of the base band MUSE signal. As illustrated, the MUSE frequency band has a range of up to about 8.1 MHz. Meanwhile, an interference beat carrier derived from the difference between the main carrier (FM signal) fc and the pilot signal fp is 8.3 (=10.6-2.3) MHz. Because the interference beat carrier is close to the MUSE frequency band, it appears as a stripe noise pattern in the picture. Where the frequency of the main carrier fc is near 14.4 MHz (i.e., when the picture appears whitish), the frequency of the interference beat carrier is 12.1 (=14.4-2.3) MHz. The latter frequency is appreciably higher than the MUSE frequency band of up to 8.1 MHz, so that the presence of the interference beat carrier becomes negligible. It follows that beat interference is insignificant when the picture is on the dark side.
Conventionally, the pilot signal (lower sideband) fp in the 2.3 MHz band is removed from the RF signal reproduced from the video disc before the main carrier fc is frequency-demodulated. However, the upper sideband fp+ contained in the reproduced signal of the video disc makes it impossible to fully suppress the beat component even after removal of the lower sideband fp.
Conventional attempts to remove the upper sideband fp+ have largely failed because the upper sideband frequency is dependent on the frequency of the main carrier fc and is thus variable. In addition, the interference beam carrier near 8.3 MHz is difficult to remove because that frequency band is close to the proper frequency band of the MUSE system.
These disadvantages of the prior art have contributed to making full suppression of the interference from the pilot signal difficult to accomplish.